A leaked memo shows the US State Department rejected a request to retain a plane for its security team in Libya, raising further questions about the protection provided for diplomats in Benghazi.

The email was sent to Ambassador Chris Stevens four months before his death, telling him that Washington was withdrawing a DC-3 aircraft used by the special forces team assigned to the embassy in Tripoli.

The State Department said it had turned down the request in May because commercial flights had resumed in Libya, adding that there was no evidence the aircraft would have prevented the four American deaths during the September 11 attack on the consulate.

However, the leaked memo, obtained by ABC News, will give impetus to Republican claims that Hillary Clinton’s department failed to provide sufficient aid to Mr Stevens and his staff despite repeated warnings they were vulnerable to attack.

The State Department’s handling of the Benghazi assault will come under further scrutiny during a Congressional committee hearing on Wednesday.

Congressman Darrell Issa, the Republican chair of the powerful House oversight committee, said earlier this week that he had evidence that the administration had ignored diplomats’ pleas for more protection but has yet to give details.